you may have some rubbing on the swaybar, but other than that, you should be good. i've seen plenty of jeeps stock running 245/75's.
i'm running 265's on my stock rims and only have minor rubbing issues on the swaybar, as well as the pinchweld seam. you shouldn't have any issues rubbing the pinchweld since you aren't going for a huge diameter, but you may find some rubbing on the swaybar since you are going wider. but really, a 255 is only 20mm wider than a 235, so you're only talking about 1/2" in each direction...that's not all that drastic.
most place will test fit a tire for you if you're on the fence. pick out a tire you want, and have them try one on before you mount them all. i think you'll be fine.
however, if you're thinking of lifting anytime soon, i'd suggest waiting to lift first. that will give you the ability to put a bigger tire under there, which undoubtedly you'll want to do as soon as you see all the real estate available inside the fender well.
